Sostenuto Puzzler

[Newton Hunt]

Some things come immediately to mind:

1: the blade bushings are too tight so it will not rotate freely.
Monkey binding between it's guides.
Spring not centered in monkey's notch.
Staple is bend, unsoldered or too tight in the monkey.

I assume you adjusted the sostenute as part of your regulation and found
it to be properly located.  If not:
    Too high
    Too far back
    Too low and rotating too far.

                    Newton